Convert MediaSourceEngine to ES6
Convert MediaSourceEngine to ES6

Issue #1157

Change-Id: Id9757ac39c37a149630c87bceacb11cdd2144f04

shaka-project/shaka-player,tvoli/shaka-player,tvoli/shaka-player,tvoli/shaka-player,tvoli/shaka-player,shaka-project/shaka-player,shaka-project/shaka-player,shaka-project/shaka-player